Certain industrial processes, such as heating a load in a furnace or generating steam in a boiler, rely on heat produced by the combustion of fuel and oxidant in a combustion chamber. The fuel is typically natural gas. The oxidant is typically air, vitiated air or air enriched with oxygen. Combustion of the fuel and oxidant in the combustion chamber causes NOx to result from the combination of oxygen and nitrogen. It may be desirable to suppress the resulting emission of NOx in the products of combustion (flue gas).
Flue gas recirculation (FGR) is known as a technique to lower NOx emission from burners. One approach is to use the combustion air blower to recycle some amount of the flue gas from the exhaust stack and to mix it with ambient air before delivery into the burner. Another approach is to use a separate blower to recycle the flue gases from the exhaust stack and introduce them into the furnace.
Some once-through steam generators (OTSGs) in the prior art employ fired burners with flue gas recirculation (“FGR”) by inducing products of combustion (“POC”) into the flame from the furnace. Some fired burners employ the FGR technique by using the POC from the exhaust system to mix with gas or fuel which reduces flame temperature. Some employ the FGR technique along with fuel staging to reduce NOx. These are often referred to as ultra-low-NOx burners (ULNBs). In ULNBs, flue gas is internally recirculated using the pressure energy of fuel gas, which dilutes the fuel/air mixture and results in lower burning rates and reduced flame temperatures and subsequently, lower NOx emission levels.
The normal solution in the prior art to reduce the NOx emission of OTSG's is by complete replacement of the burner, and installation of larger than current combustion air blower to support the need for the addition of 15 to 30% FGR, as well as additional system retrofits and installation of additional system instrumentation.
There is a need for improved FGR techniques and burners that result in optimal NOx reduction, e.g., less than 5 ppm level. There is also a need for low cost methods to retrofit existing burners, including ULNBs, for optimal NOx reduction.